Even amber appears to have some relation to coal. It is found in the unconsolidated earth in Prussia and Pomerania; but I am not sure whether this earth is travelled or not. In the same earth where the amber is found, there is often a mixture of coaly matter, which burns in the fire; it is apparently fibrous, and has been considered as a kind of fossil-wood.[84]
[84] Buffon, Hist. Nat. des Mineraux, tom. ii. p. 5.
These circumstances make out a connection between the purer bitumens and ordinary coal; but do not, it must be acknowledged, establish any thing with respect to the more immediate relation, supposed in this theory to exist between them and blind coal. It is probable, indeed, that, to discover any facts of that kind, the natural history of both substances must be more carefully examined; the natural history of blind coal, in particular, has hitherto been but little attended to.
176. A fact is mentioned by Mr Kirwan, which must not be regarded as less valuable for being adverse to this theory. It is, that neither petroleum, nor any fossil bitumen, is found in the vicinity of the Kilkenny coal, as might be expected, if that coal was deprived of its bituminous part by subterraneous distillation.[85] This, however, admits of explanation. Though a general connection, on the above hypothesis, might be expected between bitumens and infusible coal, we cannot look for it in every instance. The heat which drove off the bitumen from one part of a stratum of coal, may only have forced it to a colder part of the same stratum; and thus, in separating it from one portion of carbonic matter, may have united it to another. Blind coal may therefore be found where no bitumen has been actually extricated. In like manner, bitumen may have been separated, where the coal was not reduced to the state of coke, as a part of the bitumen only may have been driven off, and enough left to prevent the coal from becoming absolutely infusible.
[85] Geol. Essays, p. 473.
It should be considered too, if the bitumen was really separated, and forced, in the state of vapour, into some argillaceous or limestone stratum, that this stratum may have been wasted and worn away long ago, so that the bitumen it contained may have entirely disappeared. It does not therefore necessarily follow, that, wherever we find blind coal, there also we should discover some of the purer bitumens.
Note x. § 37.
The height above the level of the sea at which the marks of aqueous deposition are now found.
177. We have two methods of determining the minimum of the change which has happened to the relative level of the sea and land; or for fixing a limit, which the true quantity of that change must necessarily exceed. The one is, by observing to what height the regular stratification of mountains reaches above the present level of the sea; the other is, by determining the greatest height above that level, at which the remains of marine animals are now found. Of these two criteria, the first seems preferable, as the fact on which it proceeds is most general, and least subject to be affected by accidental causes, or such as have operated since the formation of the rocks. The results of both, however, if we are careful to select the extreme cases, agree more nearly than could have been expected.
178. The mountain Rosa, in the Alps, is entirely of stratified rocks, very regularly disposed, and nearly horizontal.[86] The highest summit of this mountain is, by Saussure's measurement, 2430 toises, or 14739 English feet, above the level of the sea, or lower than the top of Mont Blanc only by 20 toises, or 128 feet.[87] This is, I believe, the highest point on the earth's surface, at which the marks of regular stratification are certainly known to exist; for though, by the account of the same excellent mineralogist, Mont Blanc itself is stratified, yet, as the rock is granite, the stratification vertical, and somewhat ambiguous, it is much less proper than Monte Rosa for ascertaining the limit in question.